A measuring cell of the kind described above is disclosed in U.S. Pat. No. 3,655,546 incorporated herein by reference. In this measuring cell, the measuring electrode is brought up to the membrane which, in turn, is tightly held by means of an O-ring and a threaded unit so as to lie upon the measuring surface of the measuring electrode. An electrolyte film is formed between the membrane and the measuring surface of the measuring electrode which communicates with the electrolyte in the electrolyte chamber. A counter electrode is mounted in the electrolyte chamber. A perforated plate or supporting grid tightly holds that portion of the membrane in position which lies on the measuring surface of the measuring electrode. This supporting grid is intended, for example, to prevent an overpressure developing within the electrolyte from changing the spacing between the membrane and the measuring surface. Such an overpressure can, for example, develop when the measuring cell is subjected to an underpressure. Another reason for the overpressure in the electrolyte is the formation of bubbles during the measuring operation. A change of the electrolyte layer between the membrane and the measuring electrode would introduce a change in the measuring characteristics of the electrochemical measuring cell.
The disadvantage of the known arrangement of the individual parts of the electrochemical measuring cell is that care must be taken to place the membrane cleanly and tightly on the measuring surface of the measuring electrode. The same degree of care must be taken in the work procedure of tensioning the membrane. In the covered condition, the portions of the membrane outside of that portion which is pressed against the measuring surface are subjected, without protection, to the overpressures which can possibly develop in the electrolyte chamber. The pressure forces which are thereby developed can at least partially bulge the membrane and even perforate the same. During transport, acceleration forces can act on the electrolyte which can operate on the unprotected part of the membrane and cause this part to tear. Volume changes of the electrolyte caused by temperature influences, gas formation, fluid absorption or drying cannot be compensated for by the membrane.